This invention relates to fixation screws for use in orthopedic applications.
It is well known that fractured bones may be effectively healed by fixing the fractured bone in a sufficiently secure position to prevent slippage or separation. With a bone so secured, bone tissue will grow and bone cells will multiply in the region of the fracture. One common technique for securing fractures is an external fixation pin or screw which extends into or through the bone fragments to secure the fragments in a fixed position. Typically, the fixation pin pierces the outer cortex of the fractured bone, crosses the medullary canal and embeds in the opposite cortex.
Early fixation pins were smooth, cylindrical shafts which were passed through predrilled holes. These pins had no thread to engage the bone fragments. Instead, the pin merely minimized slippage or separation. More recently, fixation screws have been employed to threadably engage the bone fragments to more securely fix the fragments in position. Fixation screws have been developed which include drilling faces and self-tapping threads with tapered and/or cutting threads.
The drilling feature of a fixation screw is accomplished through use of a wedge-shaped spade surface with knife edges which scrape away the bone upon rotation of the shaft. Self-tapping is accomplished by tapering the distal end of the thread at the drill point. Cutting threads are formed with flutes forming cutting faces at each convolution of thread, particularly in the tapered, self-tapping section.
The fixation screw often included a cannula through the shaft to permit a guide pin, mounted in a guide hole in the bone, to facilitate accurate positioning of the fixation screw. Cannulated self-drilling, self-tapping fixation screws represented a significant advance to orthopedic fixation techniques. A more thorough discussion of such devices may be found in the Stednitz U.S. Pat. No. 4,537,185, assigned to the same assignee as the present invention. One of the features of prior fixation screws resides in the fact that excess heat was not built up which could kill bone tissue. Moreover, many such pins were constructed of titanium which is sufficiently porous at its exterior surface as to permit bone growth to extend into the surface, thereby providing a more effective stabilization and healing of the bone. While these features enhanced and promoted the healing process, it sometimes occurred that the bone tissue would so thoroughly conform to the fixation screw as to make it difficult to remove the screw after the bone has healed.
Previously, fixation screws were removed or withdrawn from the fixing position by rotating the screw in a direction opposite to its insertion direction so that the thread reacts against the threaded channel in the bone to withdraw from the bone. However, if the bone has grown over the thread or into the porous surface of the screw (in the case of titanium screws), reliance on the pin shaft and existing threads to clear an opening for retraction of the screw has not always been possible. Moreover, the threaded portion of a fixation screw is usually only at the distal end of the screw which is embedded in the hard cortex on only the far side of the bone. The smooth shank extends through the outer cortex of the near side of the bone. The shank diameter is typically equal to or smaller than the root diameter of the thread, so that outer cortex bone growth in the near side of the bone conforming to the smooth surface of the shank is not easily penetrated by the withdrawing thread. More particularly, during withdrawal, the proximal end of the thread engages the cortex in the medullary canal. Since the cortex material is quite hard, the withdrawing screw often cannot easily penetrate the near cortex. Application of a greater withdrawing force can result in a free rotation of the screw in the medullary canal without entering the near cortex. If this occurs, an extractor tool must be attached to the screw to aid in its withdrawal.
Attempts have been made to employ a reverse self-tapping thread at the proximal end of the threaded portion of the bone screw to aid in the entering of the near cortex. The reverse self-tapping thread have heretofore met limited success. More particularly, prior self-tapping threads employed reverse flutes to form reverse cutting faces to cut into the bone during withdrawal of the screw. The flutes were formed by a 90.degree. cut into the thread transversing several convolutions at the proximal end of the thread, the cut forming cutting faces with a base surface tangential to the minor, or root, diameter of the thread at the base of the cutting faces. However, the geometry of the flutes was such that the bone material was often first engaged by the thread portions forwardly (during reverse rotation to withdraw the screw) of the base surfaces of the flutes. Consequently, only a small portion of the reverse cutting faces formed by the flutes engaged the bone material, and the cutting faces were of minimal effectiveness. Moreover, the flutes forming prior reverse cutting threads formed a volume adjacent the screw surface which was too small to adequately collect bone chips created during the reverse cutting process. Also, it has not been possible to provide an effective taper to reverse self-tapping threads.